1. Field of the Invention
The present invention relates to a flat panel display device, and in particular, to a display device using an insulating substrate with active elements formed in a band-like polycrystalline semiconductor film, obtained by reforming an amorphous or granular polycrystalline semiconductor film formed on the top surface of the insulating substrate so as to expand crystal grains into a substantially band-like shape by use of annealing with laser light (also referred to merely as laser hereinafter) irradiated thereto to, a process of fabricating the same, and an apparatus for fabricating the same.
2. Description of Related Art
This type of display device comprises a multitude of data lines (drain lines for thin-film transistors) juxtaposed so as to be extended in a direction of the display region of an insulating substrate on one side (hereinafter referred to also as an active matrix substrate, and as a thin-film transistor substrate in the case where thin-film transistors are used as the active elements), with active elements such as thin-film transistors, thin-film diodes, and so forth, formed thereon, a multitude of scanning lines (gate lines for thin-film transistors) juxtaposed so as to be extended in another direction crossing the direction described as above, active elements formed of a granular polycrystalline silicon film (polysilicon film) as a semiconductor film formed on the active matrix substrate and disposed at cross-over points of the data lines and the scanning lines, and pixels arranged in a matrix form, made up of pixel circuits having pixel electrodes driven by the active elements, respectively. There will be described hereinafter mainly of a display device using a silicon film as the semiconductor film and thin-film transistors, which are typical active elements, as the active elements.
With a flat panel display device of the present day, pixel circuits comprising thin-film transistors, respectively, are made up of a non-crystalline silicon film (hereinafter referred to also as amorphous silicon film) and a granular polycrystalline silicon film (hereinafter referred to also as polysilicon film) as a semiconductor film on top of an insulating substrate, made of glass, fused glass, etc., serving as an active matrix substrate, and pixels are selected by switching of the respective thin-film transistors of the pixel circuits, thereby forming images.
The thin-film transistor constituting each of the pixel circuits is driven by a driving circuit (hereinafter referred to also as driver circuit or driver) mounted on the periphery of the active matrix substrate. The granular polycrystalline silicon film described above is a silicon film which crystal grains are small in diameter as will be described later. Herein, “crystal grains are small in diameter” means a size so small that, for example, there exist a multitude of grain boundaries of silicon crystals in an active layer (or active region) of the thin-film transistor, that is, within a so-called channel width, thereby causing current passing through the active layer to cut across the multitude of the grain boundaries of the silicon crystals without fail.
If it becomes possible to form the driver circuit for driving the thin-film transistors of the pixel circuits concurrently with the thin-film transistors of the pixel circuits, drastic reduction in production cost and enhancement in reliability can be expected. However, because the conventional polysilicon film, which is a semiconductor layer for forming the active layer of the thin-film transistor, has poor crystallinity (crystal grains are small in grain size), operation performance (operation characteristic) represented by mobility of electrons or holes is low, so that it is difficult to fabricate a circuit of which high-speed and high function are required. In order to fabricate the circuit having high-speed and high function, high-mobility thin-film transistors are required, but to implement this, there is the need for improving the crystallinity of the polysilicon film. Improvement on the crystallinity means primarily expansion of the grain size of the crystal grains or rendering a dimension in one direction of the crystal grains to be greater than a dimension thereof, in other directions, so as to be turned into a band-like or stripe like shape, thereby increasing the dimensions thereof. Herein, to differentiate from the conventional polysilicon film, the silicon film as reformed is referred to a band-like polysilicon film.
As a method for improving the crystallinity of a polysilicon film, there has since been known annealing with laser light such as excimer laser, and so forth. With this method, by irradiating, for example, excimer laser to an amorphous silicon film formed on top of an insulating substrate (also referred to merely as a substrate hereinafter), made of glass, fused glass, etc., the amorphous silicon film is turned into a polysilicon film, thereby improving the mobility. However, the polysilicon film obtained by irradiation with the excimer laser is on the order of several 100 nm in grain size, and the mobility thereof is on the order of 100 cm2/Vs, so that the performance thereof is insufficient for use in the driver circuit for driving a liquid crystal panel.
As a method of overcoming the problem, annealing techniques with the use of continuous-wave laser as described in Non-patent Document 1. Patent Document 1 has description to the effect that by maintaining the pulse width of pulse laser light in a range of 1 μs to 100 ms, it is possible to reduce fluctuation in threshold value of transistor fabricated. Further, description concerning reformation of a silicon film by irradiation of laser light is given in Patent Document 2.
[Non-patent Document 1]
    F. Takeuchi et al “Performance of poly-Si TFTs fabricated by a Stable Scanning CW Laser Crystallization” AM-LCD '01 (TFT4-3)[Patent Document 1]    JP-A No. 335547/1995[Patent Document 2]    JP-A No. 283356/1993